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CHEMISTRY OF ALKANES

1. Chemistry of Alkanes

Introduction 

Alkanes are referred to as saturated hydrocarbons, that is, hydrocarbons having all carbon atoms bonded to other carbon atoms or hydrogen atoms with sigma bonds only.

As the alkanes possess weak Van Der Waals forces, the first four members, C1 to C4 are gases, C5 to C17 are liquids, and those containing 18 carbon atoms or more are solids at 298 K. They are colourless and odourless.

Physical Properties of Alkanes

1. Structures of Alkanes

All the carbon atoms present in an alkane are sp3 hybridised that is, every carbon atom forms four sigma bonds with carbon or hydrogen atoms. General configuration of alkane is CnH2n+2. They exhibit tetrahedral geometry with a bond angle of 109.47° between them.

The methane molecule has a symmetrical tetrahedral structure.

Structures of Alkanes 1

2. Solubility of Alkanes

  • Due to very little difference in electronegativity between carbon and hydrogen and the covalent nature of C-C bond or C-H bond, alkanes are generally non-polar molecules.
  • As we generally observe, polar molecules are soluble in polar solvents whereas non-polar molecules are soluble in non-polar solvents. Hence, alkanes are hydrophobic in nature, that is, alkanes are insoluble in water.
  • However, they are soluble in organic solvents as the energy required to overcome the existing Van Der Waals forces and the energy required to generate new Van Der Waals forces is quite comparable.

3. Boiling Point of Alkanes

As the intermolecular Van Der Waals forces increase with the increase of the molecular size or the surface area of the molecule we observe,

  • The boiling point of alkanes increases with increasing molecular weight.
  • The straight-chain alkanes are observed to have a higher boiling point in comparison to their structural isomers.

4. Melting Point of Alkanes

  • The melting point of alkanes follows the same trend as their boiling point, that is, it increases with an increase in molecular weight.
  • This is attributed to the fact that higher alkanes are solids and it’s difficult to overcome intermolecular forces of attraction between them.

Chemical Properties of Alkanes

Alkanes are the least reactive type of organic compound. Alkanes are not absolutely unreactive. Two important reactions that they undergo are combustion, which is the reaction with oxygen and halogenation, which is the reaction with halogens.

1. Combustion

A combustion reaction is a chemical reaction between a substance and oxygen that proceeds with the evolution of heat and light. Alkanes readily undergo combustion reactions when ignited. When sufficient oxygen is present to support total combustion then carbon dioxide and water are formed.

CH4 +2O2 →CO2 +2H2O+energy

2C6H14 + 19O2 → 12CO2 + 14H2O + energy 

The exothermic nature of alkane combustion reactions explains the extensive use of alkanes as fuels. Natural gas which is used in home heating is predominantly methane.

2. Halogenation

Halogenation of an alkane produces a hydrocarbon derivative in which one or more halogen atoms have been substituted for hydrogen atoms. An example of an alkane halogenation reaction is

CH3-CH3 + Br2  →  CH3-CH2-Br + HBr

CH4 +2O2 →CO2 + 2H2O+energy

2C6H14 + 19O2 → 12CO2 + 14H2O + energy

The exothermic nature of alkane combustion reactions explains the extensive use of alkanes as fuels. Natural gas that is used in home heating is predominantly methane.

Alkane halogenation is an example of a substitution reaction, a type of reaction that often occurs in organic chemistry.

A general equation for the substitution of a single halogen atom for one of the hydrogen atoms of an alkane is

R-H + X2 → R-X + H-X

 

3. Aromatisation

At high temperatures and in presence of the catalyst, alkanes having six to ten carbon atoms are transformed into the homologous benzene. Aromatization is the term for this process. It occurs when alkanes undergo simultaneous cyclisation and dehydrogenation.


4. Reaction With Steam

In the presence of nickel, methane combines with steam at 1273K and decomposes into carbon monoxide and hydrogen gas.



Steam reforming is a well-established industrial technique for producing H2 gas from hydrocarbons, and it is used to produce H2 gas from methane.

5.Pyrolysis

Pyrolysis is described as the application of heat to decompose an organic component into smaller pieces in the absence of air. ‘Pyro’ is Greek for ‘fire,’ and ‘lysis’ is Greek for separating.’ Alkane pyrolysis is also known as cracking.

When alkane vapours are pushed over red-hot metal in the absence of air, they decompose into simpler hydrocarbons.



Preparation of Alkanes

1. Hydrogenation: Alkane can be prepared from alkene and alkyne through the process of hydrogenation. In this process, dihydrogen gas is added to alkynes and alkenes in the presence of a catalyst. These catalysts are finely divided and may include nickel, palladium or platinum to form alkanes. With the help of nickel as the catalyst, this reaction takes place at an elevated temperature, whereas it takes place at room temperature with platinum as the catalyst.

a.Hydrogenation of Ethene


b. 


c. 

 

2. Preparation of Alkanes from alkyl halides:

Alkane can be produced from alkyl halides predominantly by two ways:

i. Alkanes can be prepared from alkyl halides (except fluorides) through reduction with zinc and dilute hydrochloric acid.

a. CH3-Cl + H2 → CH4 + HCl


b.  


3. Wurtz reaction

In dry ethereal solution, on treating alkyl halides with sodium metal, the production of alkanes is higher. By this reaction, we can achieve higher alkanes with an even number of carbon atoms.

a. CH3-Br + 2Na + BrCH3 → CH3-CH3 + 2NaBr

b. 

4. Preparation of alkanes from carboxylic acids 

Decarboxylation: Alkanes can be prepared from carboxylic acid via the removal of carbon dioxide. This process is known as decarboxylation Sodium salts of carboxylic acids (RCOONa) on heating with soda lime (NaOH + CaO) form alkanes containing one carbon atom less than the salt.. It produces alkane with a carbon atom lesser than that present in the carboxylic acid.


5. From Aldehyde and Ketones
The carbonyl group of aldehydes and ketones can be converted into a methylene group by Clemmensen and Wolff-Kishner reductions.

Types of Alkanes

1. Unbranched Alkanes

Alkanes with unbranched carbon chains are also known as normal alkanes or n-alkanes.

The first four n-alkanes are Methane, Ethane, Propane, and Butane.

 

2. Branched Alkanes

As the number of carbons of an alkane increase beyond three, the number of possible structures increases. An alkane with molecular formula C4H10, for example, has two different ways to connect the carbon atoms.

This gives rise to the concept of isomer in alkanes known as the structural isomerism.

 

Isomers of Alkanes

1. Isomers of Pentane

 

 

 

 

2. Isomers of Hexane

 

The number of theoretically possible isomers increases sharply as the homologous series expands.

Study this table below:

Numbers of Carbon (n)

Possible number of Isomers

5

3

6

5

7

9

8

18

9

35

10

75

20

366319


Uses of Alkanes

  1. Natural gas is made up of methane. In both households and enterprises, LPG (a combination of butane and isobutane) is utilised as a fuel.
  2. Carbon black, which is used in printing inks, painting, and automotive tyres, is made from alkanes.
  3. Alcohols, aldehydes, and carboxylic acids are produced via catalytic oxidation of alkanes.
  4. Gasoline, kerosene oil, diesel, lubricating oils, and paraffin wax are all examples of higher alkanes.
  5. Certain halogen derivatives, such as chloroform and carbon tetrachloride, are made from alkanes and are used as solvents in industry and laboratories.